Transcript Hydraulic Pumps - catatan si boy | Terus Berkarya Hingga

Hydraulic Pumps
Positive Displacement Devices
Displacement Formulae
Characteristics
Gear Pumps
(External Gear)

Pumping Mechanism
Gear Pumps
(External Gear)
Displacement parameters and
determination
 Displacement = π/4(Do2 – Di2)L
 Do = Outer diameter of the two gears
 Di = Inner diameter of the two gears
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(Actually it is the diameter of the circle defined
by the center of one gear and the outer
diameter of the other.)
Gear Pumps
(External Gear)

Advantages:
Cheap (easy to manufacture)
 Compact
 Cheap


Did I say inexpensive?
Gear Pumps
(External Gear)

Disadvantages
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Limited pressure
capability
Unbalanced (note
where pressure is)
Results in large
bearing loads
Can be noisy (gear
mesh noise)
Volumetric efficiency?
Fixed Displacement
Gear Pumps
(Internal Gear)

Pumping
Mechanism
Gear Pumps
(Internal Gear)
Displacement is a function of the number
of teeth on the internal and external gears
and the size of the crescent divider.
 ( I don’t have a formula for the
displacement. Perhaps you can derive
one.)

Gear Pumps
(Internal Gear)

Advantages
Similar to external gear pumps in many
respects
 Quieter as gear slap is reduced

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Disadvantages
Somewhat more difficult to manufacture
 Same issues of volumetric efficiency
 Same issues of unbalanced forces
 Fixed displacement

Gear Pumps
(Internal Gear - Gerotor)

Mechanism

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External (inside)
gear is shaft driven
Internal gear is
driven by external
Single tooth space
is displaced
Design keeps
tolerance close
throughout the cycle
Gear Pumps
(Internal Gear - Gerotor)

Advantages
Cheap
 Simple
 Cheap

Gear Pumps
(Internal Gear - Gerotor)

Disadvantages
Limited pressure capability
 Unbalanced design
 Fixed displacement
 Frequently used as a charge pump

Vane Pumps

Pumping
mechanism
Vane Pumps

Displacement

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VD = π/2(Dc-DR)eL
C = Cam
R = Rotor
E = eccentricity
L= depth
Vane Pumps
(Variations)

Vane tip pressure control options
Outlet pressure under the vanes
 Surface pressure under the vanes
 Intravanes: outlet pressure is applied always
to a small area of the vane while surface
pressure is applied to the rest of the area
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These are probably Vickers innovations
and hence are highlighted in the text
Vane Pumps
(Variations)

Balanced designs
Vane Pumps
Advantages

Cartridges to quickly replace rotating group
Vane Pumps
(Variations)

Variable Displacement Design
Vane Pumps

Advantages
Quieter than gear pumps
 Higher pressure capability than gear pumps?
 Better volumetric efficiency than gear pumps?
 Can be balanced in design for longer life
 Variable displacement an option

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Disadvantages

More complex and expensive than gear
pumps
Piston Pump Designs

Axial Piston
Piston Pump Designs

Displacement of an axial piston pump
VD = YAD tan(θ)
 Y = Number of Pistons in the rotating group
 A = the area of a single piston
 D = is the diameter of the centerline circle of the
piston bores
 θ is the angle of the swashplate or the bend angle
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Piston Pump Designs
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Radial piston design
Piston Pump Designs
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Bent axis design
Piston Pump Designs
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Bent axis – variable displacement design
Piston Pump Designs
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Axial piston – variable displacement design
Piston Pump Advantages
Generally highest volumetric efficiency
 Generally highest pressure capability
 Variable displacement designs

Piston Pump Disadvantages

Higher cost (complexity)
General Issues
Pumps are not strictly continuous flow
devices. Discrete chambers are involved.
 Flow is collected for discharge through
valve plates
 Design of the valve plate and the pump
mechanism affects pressure pulses and
variation (ripple) of torque and pressure
 Design of pumps is not taught here

General Issues
Our theoretical displacements can be used
to determine theoretical pump flow
 Actual flow is a linear function of pump
displacement, speed, a units constant, and
an efficiency term
 Two kinds of inefficiencies
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Volumetric losses
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Friction losses
Actual Pump Output, Q
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Qp = (Vp np ηVp) /1000
where:
Q: L/min
Vp : cm3/rev
ηVp: Volumetric efficiency (decimal)
OR… Qp = (Vp np ηVp) /231 where:
Q: GPM
Vp: in3/rev
ηVp: same as above (no units)
Torque to Drive a Pump

Tp = (ΔP Vp)/(2π ηtp)
where:
Tp : Newton meters torque required
 ΔP : pressure rise across the pump in MPa
 Vp : Pump displacement in cm3/rev
 ηtp : Pump torque efficiency – a decimal
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OR…
Torque to Drive a Pump
English Units

Tp = (ΔP Vp)/(2π ηtp)
where:
Tp : inch lbs torque required
 ΔP : pressure rise across the pump in PSI
 Vp : Pump displacement in inches3/rev
 ηtp : Pump torque efficiency – a decimal
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Power to Drive the Pump

The hydraulic power is QpΔP/60 or
QpΔP/1714 for SI and English units
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(note this is actual pump flow, not theoretical)
Shaft power to drive the pump is given by
Psp = Phydr / ηpp where:

ηpp = ηvp ηtp which is total pump efficiency
What Determines ηvp & ηtp ?
ηvp is a function of clearance spaces, system
pressure, and pump speed
 Leakage flow at a given pressure is relatively
fixed regardless of pump speed
 It is also affected by fluid viscosity as lower
viscosity fluid will result in higher leakage
flow and lower volumetric efficiency

What about Torque Efficiency?
Torque efficiency is a function of speed
and fluid viscosity
 Higher pump speeds will result in lower
efficiency as viscous friction is speed
dependent
 Lower viscosity fluid can reduce viscous
losses but acts negatively on volumetric
efficiency

Efficiencies
(μ n)/(ΔP x 1000)
Sizing Pumps

Component sizing begins with the LOAD

Load and actuator will determine
 Flow
requirement for this circuit
 Pressure range required by the circuit
 (We’ll do this with cylinders and motors… soon)
Total the simultaneous flow requirements
 Select for the maximum load pressure
 Add pressure drops that will occur in valves,
lines and fittings ( another topic to come…)
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Pump Sizing
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With pump outlet pressure and flow known
we will consider speed.
Industrial apps will use synchonous speed of
electric motors. Generally 1750 rpm, or
possibly 1100. ($ decides)
 Small diesel apps such as skid loaders can
operate directly from engine crankshaft and
will have engine speed. (2000-3000 rpm).
 Larger diesel apps – pump splitter with gear
reductions possible to optimize speed
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Pump Sizing
Determine appropriate speed for your app
 Use the equation for pump flow, solved for
displacement
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Vp = 1000Q/p (np ηVp)
What shall we use for ηVp??
This is a function of speed, pressure, and fluid
viscosity
 Look for vendor data or curves and adjust…
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Example Pump Problem
Car Crusher
Need 125,000 lbs of force
 8 foot stroke
 10 seconds to extend?
 Target system max pressure of 1500 psi
 What is the cylinder size needed?

125,000 lbs/ A (area) = 1500 psi
 Area = 83.33 in2
 πr2 = 83.33 in2 r = 5.15 inches (let’s use 5”)
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Car Crusher Pump cont’d
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What will the system pressure be?
Cylinder area = 52 π = 78.53 in2
 125,000 lbs / 78.53 in2 = 1592 psi
 We study our plumbing and valves and allow
for 300 psi drops in our system
 Set PRV to 1900?
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Car Crusher Pump cont’d
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What is flow is required of the pump?
Q = cyl stroke x area /time
 Q = 96 in x 78.53 in2/ 10 sec = 754 in3/sec
 754 in3/sec x 1 gal/231 in3 x 60 sec/min
 Q = 195.8 GPM

 Note
that we have sized for one cylinder. We
might have others (a cylinder to kick your crushed
Hummer bale out of the machine). Size for those
that will be used simultaneously.
Car Crusher Pump cont’d
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Pump speed:
Electric power available? - 1750 rpm
 Remote from grid? Diesel at 2200 rpm
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Determine approximate size
Vp = 1000Q/p (np ηVp) or 231Q/p (np ηVp)
 Vp = 231*196/(1750*.95)
 Vp = 27.2 inches3/revolution

Car Crusher Pump cont’d
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Large pump (27.2 in3/rev)
Now we would look at vendors
 For this large, a piston design is likely
 Could also select two or more smaller pumps
operating in tandem with outlets coupled
 Selection will be based upon costs of
installation, costs of operation, and required
life

 Continuous
use favors efficiency
 Intermittent use may favor low initial cost
Pumps Selection

Fixed or variable displacement?
So far our circuit is simple and we would likely
use a fixed displacement pump
 Later we will look at more efficient circuits and
may wish to select a variable displacement
pump with appropriate controls

Positive displacement pumps:
Reciprocating
piston
Double
screw pump
Three-lobe pump
(left)
Double
circumferential
piston (centre)
External
gear pump
Sliding vane
Flexible
tube
squeegee
(peristaltic)
Pumps in series and parallel
Series
Equivalent pump
Parallel
Equivalent pump
Pumps in Series
Add the heads (H)
at each flow rate
(Q)
For example, for
two identical pumps
the head will be
double that of a
single pump.
Pumps in Parallel
Add the flow rates
(Q) at each head
(H)
For example, for
two identical pumps
the flow rate will be
double that of a
single pump.
Pump-system operation
System resistance (losses) curves
(typically H  Q2)
C = operating point
Positive Displacement Pumps

Typical Characteristics
Constant Flow at Various Pressures
 Pulse Flow is possible
 Most can pump solids suspended in liquids
 Self-priming
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Types of PD Pumps
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Rotary Pumps
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Gear – Internal, External
Lobe
Vane
Screw
Reciprocating Pumps
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Piston
Plunger
Diaphragm
Rotary vs. Reciprocating Pumps
Rotary pumps transfer liquid through the
action of a rotating mechanism (gear, lobe
or vane) operating inside a rigid container
 Pumping rates varied by changing speed
of rotor
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Rotary vs. Reciprocating Pumps
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Reciprocating pumps
move liquids by
changing the internal
volume of the pump
Require valves on the
suction and discharge
sides
Pumping rates varied
by changing the
frequency or the stroke
length
Source: http://www.watson-marlow.com/wna-se/p-fmi.htm
Internal Gear Pumps
•Smaller gear rotating
within a bigger gear
•Partial vacuum created
by meshing and
unmeshing of internal
teeth with external teeth
•Crescent divides liquid
flow between rotor and
Source:
http://www.pumpschool.com/principles/internal.htm
PD Pump Curve
Source: http://www.driedger.ca/ce2_pdp/CE2_PDP.html